Generally, examples of a high-temperature heat insulating component having high strength include the following techniques.
(1) Thermal spraying of ceramics onto the surface of a metal matrix (e.g., official gazette of Laid-Open Japanese Patent Publication No. 61-169241) PA0 (2) Making components from ceramics (I)--Application of silicon nitride ceramics (e.g., Journal of Materials Science, Vol. 15, p. 2661 (1980)) PA0 (3) Making components from ceramics (II)--Addition of a second component in the reaction sintering of Si (e.g., J. Ceram. Soc. Jpn., 102 [6], pp. 598-602 (1994))
The object of this technique lies in improving heat resistance and heat insulating properties by thermal-spraying ceramics onto the surface of the metal matrix; however, there exist problems to be dissolved such as problems of poor strength of a thermal-sprayed film itself, release between a metal matrix and the thermal-sprayed film and complicated processes needed.
The object of this technique lies in accomplishing high strength and heat resistance of the components; high strength of them has been accomplished (about 1 GPa), but there exists a problem of not so lowered thermal conductivity (about 30 to 40 W/mK).
The object of this technique lies in making silicon nitride ceramics having low thermal conductivity; lowering of the thermal conductivity of them has been accomplished (2 to 4 W/mK), but there exists a problem of not so high strength (about 200 MPa), which should be dissolved.
Here, further looking over prior arts related to silicon nitride ceramics, silicon nitride has strong covalent bonding properties and excellent stability at a high temperature as compared with oxide ceramics; and hence, the silicon nitride has been developed as a high-temperature structural material, and it has begun to be used as a part for engine including an automobile turbo-charger recently. Though conventional silicon nitride ceramics having high strength have succeeded in obtaining high strength, thermal conductivity thereof is comparatively high as 30 to 40 W/mK, as described above, and hence they have failed to be used as a heat insulating structural component as yet.
Generally, it is effective for lowering thermal conductivity of ceramics to disperse pores therein. It is known, however, that if pores are dispersed in a matrix composed of isotropic particles, strength of them are lowered fairly. For example, according to J. Am. Ceram. Soc., 73 [9], pp. 2684-89 (1990)), presence of 10% of pores in the matrix lowers strength of them by about 40%.
As described above, it has been hard to prepare a material having both high strength and low thermal conductivity according to prior techniques. It is the object of the present invention to provide porous silicon nitride ceramics having high strength and low thermal conductivity and a method for production thereof.
The present inventors have engaged in studies about controlling the microstructure of silicon nitride ceramics prior to the present invention, and as a result have found that a sintered body having a microstructure with large elongated grains developed from seed crystals as nuclei oriented parallel to the casting plane is obtained by adding a small amount of single crystal beta-silicon nitride rodlike particles into a raw powder as seed crystals and sintering a formed body with seed crystals oriented by a sheet-casting technique, and that the sintered body has both high strength and high toughness in the direction vertical to the orientation direction, and has filed a patent application (Japanese Patent Application No. 6-336177/1994). Moreover, as a result of further performing the studies, the present inventors have found that, in the case of increasing the amount of seed crystals to be added, a porous sintered body with large elongated grains being complicated, developed from seed crystals and having a microstructure with fine pores sealed among the elongated grains can be obtained, having pores being introduced without lowering strength of them; that is, the present inventors have succeeded in developing a technique of introducing pores into a microstructure of silicon nitride ceramics, while maintaining the high strength of them, which has led to the accomplishment of the present invention of a porous silicon nitride sintered body having both high strength and low thermal conductivity.